[CodeCompletion] Fix a crash in context type analysis for tuple expr

When completing inside tuple expressions, the context tuple type may
have fewer number of elements. In such cases, we cannot provide the
expected type.

rdar://problem/61668779
(cherry picked from commit 47e303a)

Author: rintaro

lib/IDE/ExprContextAnalysis.cpp

@@ -695,7 +695,9 @@ class ExprContextAnalyzer {
       unsigned Position = 0;
       bool HasName;
       if (getPositionInArgs(*DC, Parent, ParsedExpr, Position, HasName)) {
-        recordPossibleType(tupleT->getElementType(Position));
+        // The expected type may have fewer number of elements.
+        if (Position < tupleT->getNumElements())
+          recordPossibleType(tupleT->getElementType(Position));
       }
       break;
     }

test/IDE/complete_call_arg.swift

@@ -94,6 +94,9 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=ARCHETYPE_GENERIC_1 | %FileCheck %s -check-prefix=ARCHETYPE_GENERIC_1
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=PARAM_WITH_ERROR_AUTOCLOSURE| %FileCheck %s -check-prefix=PARAM_WITH_ERROR_AUTOCLOSURE
 
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TUPLEELEM_1 | %FileCheck %s -check-prefix=TUPLEELEM_1
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=TUPLEELEM_2 | %FileCheck %s -check-prefix=TUPLEELEM_2
+
 var i1 = 1
 var i2 = 2
 var oi1 : Int?
@@ -751,3 +754,14 @@ struct TestHasErrorAutoclosureParam {
 // PARAM_WITH_ERROR_AUTOCLOSURE: End completions
   }
 }
+
+func testTupleElement(arg: (SimpleEnum, SimpleEnum)) {
+  testTupleElement(arg: (.foo, .#^TUPLEELEM_1^#))
+// TUPLEELEM_1: Begin completions, 3 items
+// TUPLEELEM_1-DAG: Decl[EnumElement]/ExprSpecific:     foo[#SimpleEnum#]; name=foo
+// TUPLEELEM_1-DAG: Decl[EnumElement]/ExprSpecific:     bar[#SimpleEnum#]; name=bar
+// TUPLEELEM_1-DAG: Decl[EnumElement]/ExprSpecific:     baz[#SimpleEnum#]; name=baz
+// TUPLEELEM_1: End completions
+  testTupleElement(arg: (.foo, .bar, .#^TUPLEELEM_2^#))
+// TUPLEELEM_2-NOT: Begin completions
+}